Dr. Rita Nahta's career goal is to become an independent researcher focused on the molecular mechanisms of drug resistance in breast cancer with a specific interest in targeted therapeutics and growth factor receptors. This K01 Award will assist her transition to a fully independent scientific investigator. The next year or two will be critical for polishing her molecular biology skills and developing new skills in the areas of nanotechnology and animal models under the mentorship of Drs. Francisco J. Esteva and Mien-Chie Hung. The M. D. Anderson Cancer Center offers an excellent collaborative environment for implementation of the proposed research plan. Drs. Esteva and Hung are highly regarded among the scientific community, and Dr. Nahta will benefit from their combined expertise in the areas of signal transduction, drug resistance, molecular therapeutics, cell cycle regulation and translational research. The Breast Cancer Translational Research Laboratory, which is directed by Dr. Esteva, is an integral component of the Breast Cancer Research Program directed by Dr. Hung. The stimulating intellectual and scientific environment of this program will greatly enhance Dr. Nahta's career development. Dr. Nahta's current research focus is on the molecular mechanisms of resistance to the HER-2-targeted antibody Herceptin. Using an in vitro model of Herceptin resistance that she developed, she observed the following: 1) HER-2 forms a unique heterodimer with IGF-IR in resistant cells. HER-2 in this complex is phosphorylated, suggesting cross-talk from IGF-IR to HER-2;2) p27kip1, which lies downstream of both HER-2 and IGF-IR, is downregulated in resistant cells. Ectopic expression of p27kip1 restores Herceptin sensitivity. Based on these findings, the central hypothesis of this application is that Herceptin resistance is mediated by increased binding of HER-2 to IGFIR with subsequent degradation of p27kip1 in breast cancer cells. Our specific aims are to: 1) Characterize the interaction between HER-2 and IGF-IR;2) Define the molecular mechanism by which HER-2/IGF-IR downregulates p27kip1;3) Investigate HER-2/IGF-IR as a therapeutic target in vivo. We will use GST pulldown assays, nanotechnology, protein assays, PCR, siRNAtransfection, and in vivo mouse studies. Our studies will ultimately allow the identification of tumors most likely to respond to Herceptin, and will guide the development of more effective targeted therapies for HER-2-overexpressing breast cancer.